Conventional binocular displays present images to the left and right eyes of a user as created by left and right display screens. Optics relay images of the display screens, generally as virtual images, to the user's eyes. Such binocular displays are used in a wide variety of applications including, head, hand, table, or otherwise mounted displays for reproducing still or moving images in monoscopic or stereoscopic form.
One characteristic of such binocular displays that contributes or detracts from the comfort of the user is the relative optical contrast presented to the user's eyes from left and right display screens. If one of the user's eyes receives an image that has perceptively greater contrast than the image received by the user's other eye, the user may experience visual discomfort, headaches, or the like, and as a result, may be unable to use the binocular display for extended periods of time. This invention has, as one of its objects, a goal of improving contrast uniformity of such binocular displays.
It is a characteristic of certain display screens that the optical contrast varies substantially with the viewing angle. While display screens are generally thought to provide optimum contrast along an on-axis viewing direction (generally, normal to the surface of the display screen), some display screens produce optimal contrast at a slightly different viewing angle, for example twelve degrees from the on-axis direction. Moreover, the deviation from optimal contrast in some display screens is not symmetrical about the optimal viewing angle. That is, contrast may reduce gradually as the angle is increased in one direction and may decrease precipitously as the angle is increased in the opposite direction.
While it is possible to create a binocular display having the left and right display screens positioned so that uniform contrast is produced for users having a given interpupillary distance (IPD), the interpupillary distances of individual users tend to vary throughout populations of users. For example, most members of the adult population have interpupillary distances within a range from approximately 52 to 72 millimeters. The viewing angles presented to users' left and right eyes tend to vary in opposite directions with changes in the users' interpupillary distances (assuming as is ordinarily the case that the users' eyes remain evenly spaced from the center of the binocular display). It can now be appreciated that if the known binocular displays are set to produce equal contrast for an average interpupillary distance of, say, 61 millimeters, deviations from this spacing may not only produce reduced contrast but may also produce unequal changes in contrast for the left and right eyes, resulting in the physiological discomfort described above.